ABSTRACT Glycogen synthase kinase 3 (GSK-3) is an enzyme that is involved in multiple signal transduction pathways and is highly abundant in brain tissues. GSK-3 is upregulated in Alzheimer's disease (AD), and inhibition of this kinase can modulate the level of tau phosphorylation and prevent the subsequent formation of neurofibrillary tangles. Despite substantial progress in AD biomarker development, particularly with amyloid plaques, positron emission tomography (PET) imaging tracers for AD only show weak correlation with cognitive impairment and new approaches for imaging AD are needed. The central role of GSK-3 has led to the proposed ?GSK-3 hypothesis of AD?. To date, there are two major obstacles for molecular imaging of GSK-3 in the central nervous system with PET: 1) the discovery of potent and highly selective small molecules over other central nervous system targets and closely related kinases; and 2) achievement of reasonable brain penetration. We have discovered a highly promising brain-penetrating inhibitor of GSK-3 bearing a novel scaffold. Our lead compound known as PF-367 is highly potent and represents the most selective inhibitor of GSK-3 that has been reported to date. PF-367 showed good uptake in rodents and rapid decrease in pTau by inhibiting the GSK-3 pathway. This inhibitor demonstrated efficacy in modulation of tau phosphorylation in vitro and in vivo with excellent bioavailability and represents an outstanding lead diagnostic neuroimaging agent for GSK-3. The P.I. reported the first PET radiotracer for GSK-3 in 2005 and we have now synthesized a 11C- isotopologue of PF-367. Our preliminary PET imaging studies in non-human primates confirm high brain permeability of [11C]PF-367 and specificity for GSK-3 in vivo. By completion of this 4-year R01 proposal, we plan to develop this highly promising first generation GSK-3 PET radiotracer, along with closely related analogs, to be well situated for first in human PET imaging studies of GSK-3. Systematic derivatization of the PF-367 scaffold to discover 11C- and 18F-radiotracers will involve new approaches including: 1) fluorination of the non-activated arene using our new iodonium(III) ylide radiofluorination strategy; and 2) High throughput 11C-amidation via using our [11C]CO2 fixation strategy. Lead radiotracers will be evaluated by in vitro binding and autoradiography studies, in vivo PET imaging and ex vivo biodistribution studies in rodents and in vivo PET imaging in non-human primates to establish the optimal GSK-3 PET radiotracer for clinical translation.